This invention relates to a force sensor or strain gage sensor for detecting the presence of a person having a weight in a car seat, and in particular to a strain gage that can detect the presence of an occupant using strain sensitive resistors and provide an electrical signal to control activation of an airbag.
Various devices are well known for their ability to measure force, pressure, acceleration, temperature, position, etc. by using a sensing structure combined with signal processing electronics. One general type of sensor or transducer for such applications is a resistive strain gauge sensor in which force or pressure is sensed or measured based on strain placed on the resistors. Resistive strain gauges function by exhibiting changes in resistance in response to force which causes dimensional changes of the resistor. An electrical signal changes voltage level as the resistance changes.
Many types of strain gauge sensors have been designed and made commercially available. Various strain gauge sensors have proven to be generally satisfactory. Prior art sensors, however, have tended to be rather expensive and not suitable in certain applications such as sensing the presence of an occupant in an automobile seat. A sensor suitable for such an application must be compact, robust, impervious to shock and vibration and yet inexpensive.
Automobile seats can use sensors to activate air bags, which would be deployed during an accident. Injury to infants or small children from air bag deployment with excessive force is a current industry problem. A weight sensor in the seat can be used to control the deployment force during air bag activation. If a heavy person is in the seat, the airbag is deployed at full force. If a light person is in the seat, such as a child, the airbag is deployed at a slower, less forceful rate or not at all.
The strain gage resistors are connected in a wheatstone bridge configuration using four resistors. Previous sensors have placed two resistors on the top surface of a substrate and two on the bottom surface of the substrate. When the substrate is stressed, two of the resistors are placed in tension and two in compression. This configuration increases the magnitude of change in the electrical output signal measured across the bridge as the resistance in two of the resistors increases and decreases in the other two resistors. If the four resistors are all placed in tension or compression, the output signal across the bridge only changes by a small magnitude.
Manufacturing a strain gage with resistors on the top and bottom surface of the strain gage adds additional cost and expense as each manufacturing step has to be repeated on both the top and bottom surfaces. Further, when the resistors are placed on different surfaces, the individual characteristics of each resistor are more susceptible to change. For example, when resistors are screened and fired on one surface considerable processing time may pass before the resistors are deposited on another side. During this time the resistor material may run out and a new lot may have to be used, the oven firing profile may change and the equipment used to deposit the resistors may change settings. This may result in resistors with different resistances and temperature performance.
A current unmet need exists for a strain gage sensor that has a large magnitude output signal with resistors on only one surface that is readily manufacturable at a low cost.
It is a feature of the invention to provide a reliable and cost-effective weight sensor and strain gage for detecting weight.
An additional feature of the invention is to provide a strain gage for measuring a weight applied to the strain gage. The strain gage including a substrate having a center section that is adapted to flex in response to the applied weight. At least one step section is attached to the center section. The center section is located adjacent the step section. The step section is adapted to concentrate the weight applied onto the center section. At least one outer section is attached to the step section. At least one strained resistor is mounted on the center section of the substrate for generating an electrical signal in response to the substrate being stressed. At least one wing section is attached to the center section. The wing section is adapted to be out of a strain path. At least one unstrained resistor is mounted on the wing section.
Another feature of the invention is to provide a strain gage for measuring an applied weight. The strain gage includes a substrate having a tapered center section that is adapted to be stressed in response to the applied weight. At least one strained resistor is mounted on the center section of the substrate. At least one wing section is attached to the center section. The wing section is adapted to be not stressed by the applied weight. At least one unstrained resistor is mounted on the wing section. A terminal section is mounted to the center section and the wing section.
There has thus been outlined, rather broadly, the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject matter of the claims appended hereto. Those skilled in the art will appreciate that the conception, upon which this disclosure is based, may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention.